If these can be grown into fully developed sperm, which the researchers hope to do within five years, they may be useful in fertility treatments.

But experts have warned the findings from the German study should be interpreted with caution at this very early stage.And proposed new laws would ban their use in fertility treatments in the UK.

The government's recent fertility White Paper proposes a ban on using artificially created sperm or eggs in assisted reproduction.

The researchers from the Universities of Göttingen and Münster and the Medical School of Hannover isolated adult stem cells - cells that have the ability to become many types of tissue in the body - from bone marrow samples taken from male volunteers. source